The bike currently has a year old new ND starter, 450w alternator and 2 year old Pc680. I had hoped that the new ND might have sorted the problem, but it remains to a degree.

I don't really want to go to a new flywheel, the flywheels or 9 tooth are still available from Motobins at a bargain price, but then I lose the advantages of both the lightening, the engine has been balanced to suit the flywheel and I would have to find a 9 tooth gear for the ND starter.

There is an old school bike workshop machinist in Bristol called Pistonbroke, I might see what he can suggest about cleaning up the ring gear

Can anyone post a close up picture of lightly worn ring gear with an emphasis on the the starter side of the ring gear?